In two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy, NMR signals are displayed as two-dimensional spectra. Therefore, 2D NMR spectroscopy offers higher resolution and makes it easier to elucidate the spectra than the conventional NMR spectroscopy. Also, it permits the elucidation of nuclear spin-spin interaction. Further, 2D NMR spectroscopy has other advantages.
The prior art 2D NMR experiment is now described in detail by referring to FIG. 1(a), where a pulse sequence with two 90.degree..times.pulses is used. A general process of measurement by 2D NMR spectroscopy consists of a preparation period preceding the first 90.degree. pulse, an evolution period of t.sub.1, and a detection period of t.sub.2. The preparation period is necessary to maintain the nuclear spin magnetization in its appropriate initial condition. The preparation pulse, or the first 90.degree. pulse, brings the magnetization into non-equilibrium state. This state is caused to evolve in the evolution period t.sub.1. In the detection period t.sub.2 subsequent to the application of the detection pulse, or the second 90.degree. pulse, the resulting free induction decay signal is detected. The phase and the amplitude of this decay signal reflect the behavior of the magnetization taken in the evolution period t.sub.1. The period t.sub.1 is varied so as to assume n discrete values. A measurement is made at each of these discrete values, using two detection systems to detect signals which are 90.degree. out of phase with each other, i.e., by quadrature detection. A combination of m free induction decay signals FIDal-FIDam detected with one of the detection systems is used as the real part of a complex NMR spectrum S.sub.1 (t.sub.1, t.sub.2). A combination of m free induction decay signals FIDbl-FIDbm detected with the other detection system is employed as the imaginary part of the complex NMR spectrum. The obtained data contains not only the information about the behavior of the magnetization in the period t.sub.2 but also the information regarding the behavior of the magnetization in the period t.sub.1.
Referring next to FIG. 1(b), a pulse sequence which is similar to the pulse sequence shown in FIG. 1(a) except that the phase of the detection pulse is shifted by 90.degree.. Measurements are made at the same number of values of t.sub.1 as the foregoing values to obtain a complex NMR spectrum S.sub.2 (t.sub.1, t.sub.2). This spectrum, recorded on a plane, have diagonal peaks which appear symmetrically and cross-peaks also appearing symmetrically. All the peaks are mixtures of absorption and dispersion waveforms. Thus, it is difficult to analyze the spectrum.
In order to avoid this problem, a two-dimensional absorption spectrum is displayed in the directions of two frequencies .omega..sub.1 and .omega..sub.2. Thus, pure absorption 2D spectrum is obtained (D. J. States et al. Journal of Magnetic Resonance 48, 286 (1982); D. Marion and K. Wuthrich, Biochem. Biophys. Res. Commu., 113, 967 (1983)). This conventional phase detection method is next described by referring to FIG. 2.
First, the x-component S.sub.x (t.sub.1, t.sub.2) of a spectrum obtained using a pulse sequence of 90.degree.x-t.sub.1 -90.degree.y-t.sub.2 is stored in a memory. The y-component S.sub.y (t.sub.1, t.sub.2) of the spectrum obtained using the pulse sequence 90.degree.x-t.sub.1 -90.degree.y-t.sub.2 is stored in another memory. These components are subjected to Fourier transformation with respect to the axis t.sub.2. The phase .phi..sub.2 of the Fourier-transformed signals is corrected. The imaginary part of the complex spectrum is replaced with the real part of S.sub.y (t.sub.1, F.sub.2) whose phase has been shifted by 90.degree.. On the other hand, the real part of the complex spectrum is left as it is. In the practical measurement, quadrature detection is employed. The obtained spectrum S.sub.x '(t.sub.1, F.sub.2) is subjected to Fourier transformation with respect to the axis t.sub.1, resulting in a spectrum S.sub.x '(t.sub.1, F.sub.2). Then, a phase correction is made under the condition of .phi..sub.1 =0. Consequently, the real part of the obtained spectrum represents a pure absorption waveform.
In the conventional phase correction method, however, when the first phase .phi..sub.2 is corrected, the human operator's experience or intuition is needed except where the amount of correction to be made to the phase .phi..sub.2 is known by some means or other. Therefore, it is difficult to achieve a perfect phase correction. For this reason, after a phase correction is made under the condition of .phi..sub.1 =0, the spectrum S.sub.x '(F.sub.1, F.sub.2) takes the form EQU (A.sub.1 +iD.sub.1) (A.sub.2 sin .phi..sub.2 '+D.sub.2 cos .phi..sub.2 ')
as shown in FIG. 2. As a result, the component including the phase .phi..sub.2 ' remains.